The process of rolling aluminium requires lubrication in order to gain a satisfactory surface finish of the strip at higher reductions. However, even with lubrication, the rolling process generates a large amount of heat, which must be dissipated to prevent equipment overheating and the breakdown of the lubricant. Therefore additional cooling of the rolls is required. At present this has only been achieved in two ways:
A small number of mills rolling cold aluminium strip or foil use water based emulsions as the rolling coolant and lubricant. This would seem to be an ideal solution as water has a high cooling capacity, whilst the oil content can be tuned to give good lubrication properties. However, unless the water is completely removed from the strip immediately after rolling, stains are created on the strip surface, spoiling its appearance. In practise, it has been very difficult to ensure completely dry strip unless the strip exit temperature from the mill is considerably greater than 100° C. This limits the practicality of rolling and hence just a few specialist mills rolling specific products use this method.
The vast majority of mills rolling cold aluminium strip or foil use kerosene as both rolling lubricant and coolant. Kerosene was found to have the best compromise between cooling and lubricating properties without having any strip marking issues. However, kerosene is not the best lubricant or coolant and has significant fire safety, environmental and health problems associated with it.
In order to provide effective cooling with kerosene, flow rates of up to several thousand liters per minute may be required. Such volumes require expensive recirculation and filtration systems and will inevitably cause oil mist to form which requires expensive fume extraction and cleaning systems. The inventors have shown that for the purpose of lubrication alone, flow rates of less that 10 liter/minute can suffice.
In both the above solutions, banks of spray nozzles apply the fluid directly to the rolls in order to effectively cool them, whilst further separately controlled spray nozzles direct fluid on to the rolls nearer to the roll nip in order to lubricate the rolling process.
A further use for the cooling sprays is also known. One of the main challenges in the cold rolling of aluminium strip and foil is to ensure that the product is flat after rolling. Bad flatness is caused by the strip being reduced in thickness by different amounts across the width of the mill. This is caused by variations in the gap between the rolls across the mill. By varying the cooling effect across the roll's width, it is possible to impart different degrees of thermal expansion to different parts of the roll, thereby providing a mechanism to compensate for local variations in roll gap.
A number of patents (e.g. GB2012198, EP41863) exist illustrating the technology for varying the cooling rate across the width of the roll and, with the use of a flatness measuring device on the exit side of the mill, directly controlling the flatness of the rolled strip.
GB2156255 describes a process which employs separate lubrication and cooling (SLC). Banks of water jets are used to cool the rolls and effect shape control, whilst low quantities of more suitable lubricating oil are applied directly to the strip upstream of the mill.
The effect known in the aluminium industry as “tight edge” is one of the main causes of strip breaks during rolling. GB2080719 describes partial roll heating using the so called “Tight Edge inductors” (TEIs)—This technology uses the induction effect to locally heat up the mill rolls in the area of the strip edge in order prevent the under rolling of the strip edges.
This technology has been used successfully on a number of mills, however, there are significant challenges with using electrical heating devices on a mill using kerosene coolant.
In their paper “Thermal Shape Control in Cold Strip Rolling by Controlled Inductive Roll Heating”, International Conference of Steel Rolling, Japan, 1980, Sparthmann & Pawelsky, describe experiments done using a combination of water cooling jets and induction heaters to effect flatness changes during the rolling of steel strip.
Further developments in this field up to the present day have been limited to improvements in the control and resolution of the kerosene cooling effect.
Meanwhile, in other fields some work had been done in using cryogenic gases or liquids as a coolant in industrial rolling processes. Various patents have been published on this topic including DE3150996, JP2001096301, WO02/087803, U.S. Pat. No. 6,874,344.
However, all this prior work has concentrated on cooling the processed material for metallurgical and other effects.
US 2007/0175255 discloses a method and apparatus for cold rolling of a metallic rolling stock in which a number of nozzles are used to apply various combinations of lubricant emulstion or base oil, coolant and inert gas are applied to the wedge and arc areas of upper and lower rolls, for the purpose of cleaning, cooling, lubrication and rendering inert. Flatness control of a thermal working roll barrel is alluded to, however, it is described as being achieved by using a combination of inert gas and conventional coolants, which in the field of aluminium rolling implies a high kerosene flow rate with all its associated equipment and safety issues.